I haven't read the book, but it sounds like the whole point is how to avoid breaking your rod, while letting a large saltwater fish pull against the drag. If your rod is not vertical, (and you are not holding the rod at an extreme angle out to one side or the other - which would make the effective angle between rod and fish the same as if your rod was vertical) the pull is against the stiffer, stronger butt sections. You would definitely be less likely to break your rod. The big difference with tenkara is that the fish cannot pull against the drag. You would definitely be more likely to break your tippet if you were fighting a big fish with 5X tippet. You could use a heavier tippet and not break your rod from putting too great a bend in it. However, the heavier tippet would increase the chance of getting a permanently jammed rod tip, and I am 99 and 44/100 percent sure that more rods are broken from trying to free stuck rod tips than from big fish.

I don't hook a lot of big fish. I rarely get broken off (and have never gotten broken off because the rod was vertical). I do get snagged a lot, either on the far bank or in a treetop following a missed strike or long distance release. It is not uncommon that have to pull back on the rod to break the tippet because I can't reach the line. I don't want to use a tippet that is so strong it is going to jam the rod tip from breaking off a snag.